A chemical mechanical polishing (CMP) apparatus is a typical apparatus for polishing a substrate, such as a wafer. This CMP apparatus is configured to supply a polishing liquid onto a polishing pad on a polishing table while rotating the polishing table, and press a wafer against the polishing pad to thereby bring the wafer into sliding contact with the polishing pad in the presence of the polishing liquid. A surface of the wafer is polished by a mechanical action of abrasive grains contained in the polishing liquid and a chemical action of chemical components contained in the polishing liquid.
The CMP apparatus typically includes a polishing-progress monitoring device in order to detect a polishing end point of the wafer. There are several types of polishing-progress monitoring devices, one of which is a table-current monitoring device that monitors a current value of a table motor for rotating the polishing table. This table-current monitoring device is configured to detect a progress of polishing of the wafer from a change in the current value of the table motor that is necessary for rotating the polishing table at a preset constant speed. The wafer typically has a multilayer structure composed of different kinds of films. When an uppermost film is removed by a polishing process, an underlying film is exposed. As a result, a frictional force between the wafer and the polishing pad changes. This change in the frictional force is reflected as the change in the current value of the table motor for rotating the polishing table. Therefore, the table-current monitoring device can detect a removal of the uppermost film from the change in the current value of the table motor.
In a case where the uppermost film of the wafer is a conductive film, a polishing-progress monitoring device having an eddy current sensor may be used. The eddy current sensor is configured to supply a high-frequency alternating current to a coil so as to induce an eddy current in a conductive film of the wafer, and detect a thickness of the conductive film from a change in impedance due to a magnetic field of the induced eddy current. The eddy current sensor is embedded in the polishing table, and obtains a film thickness signal while scanning a surface of the wafer each time the polishing table makes one revolution. The polishing-progress monitoring device can monitor the progress of polishing of the wafer from a change in the film thickness signal.
The wafer is polished while being pressed against the polishing pad in the presence of the polishing liquid. During polishing of the wafer, the abrasive grains in the polishing liquid are accumulated on a surface (i.e., polishing surface) of the polishing pad and minute surface irregularities of the polishing pad are crushed. As a result, a polishing performance of the polishing pad is lowered. Therefore, in order to restore the surface of the polishing pad, a dresser is used.
The dresser has a dressing surface which is constituted by minute diamond particles. The dresser presses the dressing surface against the polishing pad while rotating the dressing surface, and further oscillates along a radial direction of the polishing pad to thereby slightly scrape away the surface of the polishing pad. Thus, the surface (i.e., polishing surface) of the polishing pad is restored. Such a restoration of the polishing pad with use of the dresser is referred to as pad dressing or pad conditioning.
In a case where the above-described table-current monitoring device is used as the polishing-progress monitoring device, the pad dressing may be performed during polishing of the wafer. In this case, the progress of polishing is monitored based on the current value of the table motor while polishing the wafer. However, since the dresser oscillates on the polishing pad during polishing of the wafer, the current value of the table motor changes due to an influence of the oscillation of the dresser. Therefore, the polishing-progress monitoring device may fail to accurately monitor the progress of polishing.
Furthermore, the polishing-progress monitoring device using the eddy current sensor has a following problem caused by a wafer structure. Typically, a number of devices are regularly formed on the surface of the wafer to be polished, and metal structures, such as a metal film and metal interconnects, are present under the uppermost film to be polished. These metal structures are present in each device, and are regularly arranged in one wafer. When the eddy current sensor sweeps across the surface of such wafer, the film thickness signal varies due to an influence of an arrangement of the metal structures. Therefore, the polishing-progress monitoring device may fail to accurately monitor the progress of polishing.